What soil are you using? And 1/2 gal is not a flush. A flush to remove salt build up and get PPM down would require several gallons at once.
There should be no need for nutes at this stage if using a good soil and there should be no chance of salt build up yet.
Nutes are usually only added after a plain water’s runoff drops to less than 1000PPM.
Remember, that feeding schedule is designed to sell more product. 
Feed often and flush much away every so often. Then go buy more…
My ppm was way low before this which is why it was suggested to feed. They were at around 372 and I have a water filter system in my house so the pH is at 5.4 and the ppm is only 5 something. Good to know on the flush. First time doing it all and so many ways to do it all makes my head spin. I’m sure I needed them up but trial and error I guess. No book out there that says do this when…just 5 million ways. I need a grow for dummies book specific and to the point.
I may not be a good one to copy as far as nutes go. I am a seat of my pants grower and use a varied concoction in my grows. I have some fish shit type stuff(N), bone meal(P), ashes(K), one part of a 2 part flower booster(Connoisseur brand for extra P&K), epsom salts, Cal-Mag, molasses and baking soda. My soil is a weird mix of old Fox Farm, old Sta-Green, compost perlite, sand, peat moss and dirt from my yard.
Appreciate the tag Grow Bro, @Catdaddycox good info from @Graysin. The FF trio is heavy in salts and recommend always feeding to liberal run off. I consider the liberal run off as a soft flush rinse. It’ll rinse those un-used salts away from the previous feeding keeping a healthy root zone 